Satisfying Imbalance of Structure

In his introduction to The Oxford Book of Sonnets, John Fuller makes an interesting observation:

"The sonnet is a dualistic structure, formalizing the essence of drama in statment and response. In its origin in thirteenth century Sicily, the sestet [the final six lines] of the Italian sonnet may have had a different and answering music to that of the octave [the first eight lines] (typically abbaabba cdecde or cdcdcd). Certainly its defiance of the expectation of a second stanza complementary in length to the first is crucial to the satisfying imbalance of structure (8:6)."

He is quite right, there is a change of tone, and often a change of voice in the second section of many sonnets. Sometimes the latter portion answers the first, sometimes it intensifies it, or contradicts it. Perhaps it merely changes perspectives